As pearlescent pigments, there have been conventionally known flaky aluminum powders, mica flakes and graphite flakes coated with metal oxides such as titanium dioxide and iron oxide, iron oxide particles composed mainly of alpha-iron oxide crystals, and the like. These pearlescent pigments reflect light incident upon their surfaces and shine brilliantly. Therefore, when these pearlescent pigments are blended in a paint, an ink, or a resin composition, the surfaces coated with the paint, the lines drawn with the ink or the surfaces printed therewith, or the surfaces of products molded from the resin composition exhibit unique appearances full of variety and excellent in decorative effect, in synergy with the color tones of substrates.
Accordingly, the pearlescent pigments are used widely for various applications, such as paints and coating materials for automobiles, motorcycles, OA (office automation) machines, cellular phones and home electric appliances, and inks for various printing matters or writing materials, for the purpose of improving the decorative effect in their appearances. Among these applications, the pigments particularly are required to have high weather resistance when they are used for automobiles or motorcycles because they are used outdoors. When they are exposed to ultraviolet light, titanium dioxide and/or iron oxide contained in the pigments accelerate decomposition and discoloration due to their photocatalytic activity.
In order to prevent a pigment-induced decomposition and discoloration, JP 60 (1985)-92359 A, for example, discloses a pigment in which polysiloxane and a rare earth metal compound are deposited on mica flakes coated with a metal oxide.
JP 01 (1989)-292067 A discloses a pigment in which the following hydrates (i) and (ii) are deposited on the surface of mica flakes coated with a metal oxide. The hydrates are: (i) a hydrated zirconium oxide produced by hydrolysis of a zirconium compound in the presence of hypophosphorous acid; and (ii) an hydrated oxide produced by hydrolysis of a water-soluble compound of at least one metal selected from the group consisting of cobalt, manganese, and cerium.
JP 07 (1995)-18199 A discloses a pigment in which a sheet-shaped fine substrate coated with a metal oxide is coated with a covering layer. The covering layer contains silicon dioxide, metal hydroxide, and an organic coupling agent.
JP 07 (1995)-268241 A discloses that mica coated with titanium dioxide or iron oxide further is coated with cerium hydrate and aluminum oxide.
JP 2000-505833 T discloses that mica coated with titanium dioxide or iron oxide further is coated with aluminum hydroxide (or cerium hydroxide and aluminum hydroxide) and further subjected to treatment with a silane coupling agent.
JP 2002-194247 A discloses a pearlescent pigment in which a layer composed of a hydroxide of Al or Si, a layer composed of a hydroxide of at least one selected from the group consisting of Al, Si, Ce and Zr, and a layer composed of an organic hydrophobic coupling agent are formed on a flake-shaped substrate.
Although the above-mentioned improvements have been made, conventional pearlescent pigments have not necessarily achieved a high enough level in their weather resistance and water resistant secondary adhesion (adhesion after water resistance test) to coating films composed of a cured resin composition, particularly to coating films composed of an aqueous emulsion or water-soluble cured resin composition. In addition, the random orientation of pearlescent pigment particles makes it difficult to obtain a high design property. Uniform orientation of pearlescent pigment particles in a paint spread allows the pigment to exhibit strong interference colors, which enhances its design property.
As paints containing flaky pearlescent pigments, for example, those containing, as a main binder, an epoxy resin, an acrylic resin, a polyester resin, or the like, and blended with a curing agent such as polyvalent carboxylic acid, block isocyanate, or the like, as necessary, are widely known. These paints are used in many applications such as home electric appliances, automobiles, and construction materials.
Problems in each of the following fields of applications will be described below in detail.
In the field of inks, in recent years, there has been an active movement to shift from solvent-type inks to water-based inks in terms of environmental protection and resource saving. With this active movement, the requirements for binders have become more and more strict. Generally, the properties required for water-based inks are: (1) good flowability; (2) excellent storage stability; (3) quick drying; (4) good adhesion to a substrate, particularly to a plastic film; (5) high gloss; (6) good water resistance and abrasion resistance; and so forth. Water resistant secondary adhesion to a plastic film is, among others, an important required property.
As an ink having these properties, for example, JP 55 (1980)-18469 B discloses an ink composition using, as a binder, a polymer obtained from a (meth)acrylic acid derivative.
In the field of automobile painting, a so-called “two coat one bake method” is employed. According to this method, first, a metallic base paint containing a metallic pigment is applied as a top coat layer to a coated surface that has been subjected to undercoating and intermediate coating. Next, a clear coat paint further is applied to the top coat layer by a wet-on-wet technique without curing this metallic base paint. Then, the clear coating film and the metallic base coating film are cured simultaneously.
In the two coat one bake method as described above, paints containing organic solvents mainly have been used conventionally. However, from the viewpoints of safety during coating, prevention of environmental contamination, saving of resources, and so forth, there has been a strong demand for a shift to water-based paints. Therefore, a variety of water-based paints for automobiles have been developed. For example, JP 04 (1992)-25582 A discloses a water-based metallic base paint containing, as its main components, an acidic resin having an amide group and an aqueous dispersion of a polyurethane resin having a hydrophilic group.
Some of the water-based metallic base paints used in the “two coat one bake method” contain a resin having a hydroxyl group and a carboxyl group, an amide resin (for example, an alkyl-etherified methylol melamine resin), and a pearlescent pigment. When this type of water-based paints is used, during simultaneous curing of a clear coating film and a metallic base coating film, alcohol is produced as a by-product of a reaction between a hydroxyl group and an alkyl ether group while formaldehyde is produced as a by-product of a reaction between methylol melamines. The alcohol and formaldehyde volatilize to the outside through the stacked coating films. In this case, the orientation of the pearlescent pigment particles becomes random, which reduces the flop property thereof.